The invention relates to a unitary plastics material structure comprising a first layer, a second layer, and, between the layers, a first set and a second set of elements forming a bi-planar mesh, the first elements being united with the first layer, the first and second elements being united with each other at their cross-over points, and the second elements being united with the second layer. The invention relates to a method of producing the structure, to the structure itself and to apparatus for producing the structure.
GB No. 1,352,747 has suggested forming flat structures by extruding two plies as tubular extrusions, each plie having a single set of helical elements. The plies are slit, reheated and laminated together. Using this procedure, it is not possible to form tubular products.
Regarding the formation of tubular products, U.S. Pat. No. 3,464,450 discloses putting a single high-profile helical element between an inner layer and an outer layer. It is believed that there would be difficulties in producing the tubular product disclosed.
BE No. 582,284 discloses forming a tubular product by extrusion with a layer on the inside of a bi-planar mesh. The extrudate is blown in order to form film.